Cellular communications systems are well known in the art. In a typical cellular communications system, a geographic area is divided into a series of regions that are referred to as “cells,” and each cell is served by a base station. Typically, a cell may serve users who are within a distance of, for example, 2-20 kilometers from the base station, although smaller cells are typically used in urban areas to increase capacity. The base station may include baseband equipment, radios and antennas that are configured to provide two-way radio frequency (“RF”) communications with fixed and mobile subscribers (“users”) that are positioned throughout the cell. In many cases, the cell may be divided into a plurality of “sectors,” and separate antennas provide coverage to each of the sectors. The antennas are often mounted on a tower or other raised structure, with the radiation beam (“antenna beam”) that is generated by each antenna directed outwardly to serve a respective sector. Typically, a base station antenna includes one or more phase-controlled arrays of radiating elements, with the radiating elements arranged in one or more vertical columns when the antenna is mounted for use. Herein, “vertical” refers to a direction that is perpendicular relative to the plane defined by the horizon.
In order to increase capacity, cellular operators have, in recent years, been deploying so-called “small cell” cellular base stations. A small cell base station refers to a low-power base station that may operate in the licensed and/or unlicensed frequency spectrum that has a much smaller range than a typical “macro cell” base station. A small cell base station may be designed to serve users who are within a small geographic region (e.g., tens or hundreds of meters of the small cell base station). Small cells may be used, for example, to provide cellular coverage to high traffic areas within a macro cell, which allows the macro cell base station to offload much or all of the traffic in the vicinity of the small cell base station. Small cells may be particularly effective in Long Term Evolution (“LTE”) cellular networks in efficiently using the available frequency spectrum to maximize network capacity at a reasonable cost. Small cell base stations typically employ an antenna that provides full 360 degree coverage in the azimuth plane and a suitable beamwidth in the elevation plane to cover the designed area of the small cell. In many cases, the small cell antenna will be designed to have a small downtilt in the elevation plane to reduce spill-over of the antenna beam of the small cell antenna into regions that are outside the small cell and also for reducing interference between the small cell and the overlaid macro cell.
FIG. 1A is a schematic diagram of a conventional small cell base station 10. As shown in FIG. 1A, the base station 10 includes an antenna 20 that may be mounted on a raised structure 30. In the depicted embodiment, the structure 30 is a small antenna tower, but it will be appreciated that a wide variety of mounting locations may be used including, for example, utility poles, buildings, water towers and the like. The antenna 20 may be designed to have an omnidirectional antenna pattern in the azimuth plane for at least some of the frequency bands served by the base station antenna, meaning that at least one antenna beam generated by the antenna 20 may extend through a full 360 degree circle in the azimuth plane.
As is further shown in FIG. 1A, the small cell base station 10 also includes base station equipment such as baseband units 40 and radios 42. A single baseband unit 40 and a single radio 42 are shown in FIG. 1A to simplify the drawing, but it will be appreciated that more than one baseband unit 40 and/or radio 42 may be provided. Additionally, while the radio 42 is shown as being co-located with the baseband equipment 40 at the bottom of the antenna tower 30, it will be appreciated that in other cases the radio 42 may be a remote radio head that is mounted on the antenna tower 30 adjacent the antenna 20. The baseband unit 40 may receive data from another source such as, for example, a backhaul network (not shown) and may process this data and provide a data stream to the radio 42. The radio 42 may generate RF signals that include the data encoded therein and may amplify and deliver these RF signals to the antenna 20 for transmission via a cabling connection 44. It will also be appreciated that the base station 10 of FIG. 1A will typically include various other equipment (not shown) such as, for example, a power supply, back-up batteries, a power bus, Antenna Interface Signal Group (“AISG”) controllers and the like.
FIG. 1B is a composite of several views of an antenna beam 60 having an omnidirectional pattern in the azimuth plane that may be generated by the antenna 20. In particular, FIG. 1B includes a perspective three-dimensional view of the antenna beam 60 (labelled “3D pattern”) as well as plots of the azimuth and elevation patterns thereof. The azimuth pattern is generated by taking a horizontal cross-section through the middle of the three dimensional antenna beam 60, and the elevation pattern is generated by taking a vertical cross-section through the middle of the three dimensional beam 60. The three-dimensional pattern in FIG. 1B illustrates the general shape of the generated antenna beam in three dimensions. As can be seen, the antenna beam 60 extends through a full 360 degrees in the azimuth plane, and the antenna beam 60 may have a nearly constant gain in all directions in the azimuth plane. In the elevation plane, the antenna beam 60 has a high gain at elevation angles close to the horizon (e.g., elevation angles between −10° and 10°), but the gain drops off dramatically both above and below the horizon. The antenna beam 60 thus is omnidirectional in the azimuth plane and directional in the elevation plane.